This invention relates generally to apparatus for collecting solar energy and, more particularly, to such an apparatus for concentrating a high level of solar radiation onto a relatively small collection surface.
While individual solar units are presently being used in some parts of this country for heating, air conditioning and hot water, it has generally been more economical to use cheaper alternatives such as electricity or gas. In addition to the initial cost of the solar collection equipment it has also been necessary to provide 100 percent standby capacity for use during protracted periods of bad weather. With the present incentive to conserve natural resources, past considerations in opposition to the use of solar collection equipment are no longer as impelling.
In the past, systems for utilizing solar energy have consisted, generally, of: (1) a flat-plate collector having a large glazed surface for absorbing the sun's heat, said glazed surface comprising one or more sheets of glass or a radiation-transmitting plastic film or sheet; (2) tubes or fins for conducting or directing a heat-transfer fluid from an inlet duct to an outlet and thence to an insulated storage area; (3) a metallic plate which may be flat, corrugated, or grooved, to which the tubes or fins are attached in a manner which produces a good thermal bond; (4) insulation, to minimize downward heat loss from the plate; and (5) a container or casing which holds the foregoing components and protects them. Such systems have been constructed from many different materials and in a wide variety of designs. Flat-plate collectors have been arranged in series and in parallel, and are usually deployed in large numbers together, facing south in the United States. They have been used to heat such fluids as water, water plus ethylene glycol, water plus ammonia, fluorinated hydrocarbons, air and other gases. Pumps are usually provided to circulate the heat-transfer fluid through the collector and to thereby provide space heat, hot water or air conditioning.
The most significant disadvantage of flat plate collectors results from their relatively large surface areas. Since the rate of heat loss by radiation and convention is dependent upon collection surface area, the efficiency of flat plate collectors diminishes rapidly with decreases in ambient temperatures and can even reach zero at temperatures around 0.degree. F. and below. This problem is particularly significant in the northern latitudes in which energy requirements for heating purposes are highest in the periods of lowest ambient temperature.
Several studies have shown that temperatures far above those attainable by flat-plate collectors can be reached if a large amount of solar radiation is concentrated upon a relatively small collection area. To this end, paraboloidal concentrators, similar to searchlight reflectors, have been developed to follow the apparent movement of the sun. These and other diurnal tracking devices have attained relatively high temperatures but require, in general, complicated mechanisms for their operation and can use only the direct rays of the sun, since diffuse radiation cannot be concentrated. In addition, the problems associated with the fabrication of paraboloidal reflective surfaces substantially increases the overall cost of a collection system.
The object of this invention, therefore, is to provide an improved, relatively low cost solar energy collection system that exhibits high efficiency, particularly in the northern latitudes.